In directly addressed electrochromic displays or directly addressable electrochromic displays, each pixel is connected by a separate electric conduction line to an external drive voltage source, facilitating simultaneous individual electrical control of all pixels in the display. Furthermore, when operating a directly addressed electrochromic display, or directly addressable electrochromic display, it is possible to turn on, or initiating a switch of, all the pixels simultaneously. It should be noted that the expressions directly addressed and directly addressable are used interchangeably throughout the application, and both refer to a display device that is to be addressed by direct addressing. When the number of pixels in a display is large, it is either physically impossible or impractical to connect one separate line to each pixel. To overcome this problem the pixels are commonly arranged in a matrix structure in which they are addressed by time-multiplexing techniques via row and column lines from the matrix edges. Such displays and the methods of addressing them are denoted matrix displays and matrix addressing, respectively.
The switching of electrochromic electrode materials is a faradic reaction, that is, ions must be able to move into or out from the electrode to compensate for changes in oxidation levels of the electrode material. This also means that at least one ionic species in an electrolyte must be mobile in the electrode material. In electrochromic devices where the electrochromic electrode materials also serves as a conductor, providing a lead for current to a voltage source, the mobile ion can also migrate in the electrode material. This migration may result in a reduced sharpness of the colour switch as the area switched can extend from the area defined by the location of the counter electrode or by the location of the electrolyte.
WO 2008/062149 describes lateral electrochromic displays comprising PEDOT:PSS as electrochromic material, which are illustrated in e.g. FIGS. 1-3. In operation a voltage difference is applied between the two pixel portions which are bridged by electrolyte, as can be seen in FIG. 3. This voltage difference gives rise to a colouring of the electrochromic material, the colouring being initiated at a first edge of the electrochromic material at the gap between the two pixel portions. While the voltage is applied, the colouring spreads across the material and a boarder is formed between material that has switched colour and material that has not yet switched colour. It has been seen that the border between the switched and un-switched material is not always as sharp as may be desired, in order to be able to show e.g. symbols having more complex shapes.
An inherent problem when desiring to create sharp images or symbols by this or vertical type of displays originates from the fact that ions must be mobile in both the electrolyte and in the electrochromic material itself in order to give rise to the colour change. So, initially the colour change will be seen only at the area of the electrochromic material which is covered by electrolyte.
However, the ions will migrate both vertically and laterally in the electrochromic material, and thus also outside of the electrolyte covered area. This migration of the ions results in a blurring of the switched area, and thus also of the displayed symbol.
However, there is a need to find a way of producing directly addressed display devices, capable of displaying sharper images.